Platelets are circulating anucleate blood cells imperative for hemostasis. Thrombocytopenia is a major clinical problem afflicting patients with a variety of conditions, and can be a life-threatening consequence of genotoxic treatments including chemotherapy and radiation therapy. At steady state, human adults produce close to 1 trillion platelets each day from polyploid megakaryocyte (MK) precursors. The leading paradigm of megakaryopoiesis centers on the ability of cytokines, chiefly thrombopoietin (TPO), to promote MK progenitor proliferation and MK precursor maturation. Recently, however, there has been increasing focus on the ability of the bone marrow microenvironment to support MK maturation and platelet formation. It is well established that thrombopoiesis occurs in the vascular niche, where mature MK precursors associate with sinusoidal endothelium and extrude pro-platelets into the vasculature. The chemokine stromal-derived factor-1 (SDF-1), signaling through receptor CXCR4, has been implicated in the chemotaxis of MKs toward sinusoidal vessels, however we lack a comprehensive analysis of SDF-1/CXCR4 signaling in MK lineage development and platelet production. Accordingly, the overall goals of this proposal are to better understand the microenvironmental regulation of the MK lineage by elucidating the role of the SDF-1/CXCR4 axis in megakaryopoiesis and thrombopoiesis and to use this knowledge to improve therapeutic strategies for thrombocytopenia. First, we will delineate the in vivo function of SDF-1/CXCR4 signaling in MK lineage maturation, spatial localization, and platelet production during steady-state hematopoiesis. Our preliminary data indicate that SDF-1 acutely promotes the physical association of MKs with sinusoidal vasculature and increases circulating platelets. As many thrombocytopenias derive from damage to both the bone marrow microenvironment and the platelet-producing cells of the MK lineage, we will use sublethal total body irradiation (TBI) as an injury model to define the role of SDF-1/CXCR4 signaling in MK lineage recovery from clastogenic insult. In this setting, we will also examine the ability of SDF- to ameliorate radiation-induced thrombocytopenia and enhance platelet recovery. Overall, through these studies we will gain an improved understanding of the microenvironmental regulation of megakaryopoiesis and thrombopoiesis and lay the groundwork for the potential targeting of the SDF-1/CXCR4 pathway to improve thrombocytopenia.